Microlithography is used for producing microstructured components such as, for example, integrated circuits or LCDs. The microlithography process is carried out in a so-called projection exposure apparatus comprising an illumination device and a projection lens. The image of a mask (reticle) illuminated by way of the illumination device is in this case projected by means of the projection lens onto a substrate (for example a silicon wafer) that is coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection lens, in order to transfer the mask structure to the light-sensitive coating of the substrate.
In projection lenses designed for the EUV range, i.e. at wavelengths of e.g. approximately 13 nm or approximately 7 nm, owing to the lack of availability of suitable light-transmissive refractive materials, mirrors are used as optical components for the imaging process.
One problem that arises in practice is that the EUV mirrors can have production-related geometric shape deviations of their optical effective surface (also referred to as shape accuracy errors) after their production or after the coating process has been effected, and these shape deviations can lead to a negative influence on the imaging properties of the optical system unless they are corrected.
In order to permit the correction of such effects, and also to change the optical properties of a mirror in a targeted fashion, e.g. for compensating aberrations occurring in the system, it is known to control mirror deformations during the operation of the optical system by way of e.g. thermal actuation. However, such corrective measures are frequently complicated and can furthermore prove to be no longer sufficient in the case of increasing demands in terms of the accuracy of the wavefront to be set in the optical system.
Regarding the prior art, reference is made merely by way of example to WO 2007/033964 A1, DE 103 60 414 A1 and DE 10 2004 051 838 A1.